1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more particularly, to a surface acoustic wave device including a surface acoustic wave element.
2. Description of the Related Art
In a conventional balanced-type SAW (surface acoustic wave) filter having an unbalanced-to-balanced-type filter of a first stage and a balanced-to-balanced-type filter of a second stage that are cascade-connected, signal wirings for connecting the balanced terminals of the filters are disposed between the stages and a ground pad connected to the other terminal of an unbalanced terminal IDT (interdigital transducer, comb-shaped electrode) of the first stage is disposed between the signal wirings.
FIG. 5 is a top view of a substrate of a surface acoustic wave device of a related example. The substrate 1210 is a LiTaO3 single crystal substrate and, a metal film having a fixed pattern is provided on the main surface 1212, as shown in FIG. 5. That is, a balanced-type SAW filter is provided, in which a longitudinally coupled resonator-type surface acoustic wave filter 1220 of the first stage and a longitudinally coupled resonator-type surface acoustic wave filter 1230 of the second stage are cascade-connected, a pad 1251 defines an unbalanced terminal, and pads 1252 and 1253 define balanced terminals. In an area enclosed by the filters 1220 and 1230 and wirings 1241 and 1242 for cascade-connecting the filters 1220 and 1230, a ground pad 1256 connected to an IDT 1223 including the unbalanced terminal 1251 is disposed (see, for example, Japanese Unexamined Patent Application Publication No. 2002-300004).
Furthermore, in recent years, there has been a requirement for having a balanced-to-unbalanced conversion function or a so-called balun function included in a surface acoustic wave filter used in the RF stage of a portable telephone. Lately, in particular, a longitudinally coupled resonator-type surface acoustic wave filter which provides a high-frequency wave and also which provides a balanced-to-unbalanced conversion function has become more common as a bandpass filter of the RF stage of a portable telephone.
The surface acoustic wave filter having a balanced-to-unbalanced conversion function is connected to a mixer IC (hereinafter, referred to as a balanced-type mixer IC) having a balanced or differential input and output. When this balanced-type mixer IC is used, the effect of noise is reduced and the output is stabilized. Thus, this surface acoustic wave filter is often used to improve the characteristics of portable telephones.
A surface acoustic wave filter having a balanced-to-unbalanced conversion function may include various structures. Each of the various structures has benefits and detriments and may be properly used in accordance with the intended uses and requirements. One known structure includes balanced terminals that are connected to both terminals of one IDT.
For example, in FIG. 6, an element chip 30 of such a surface acoustic wave filter is schematically shown. The surface acoustic wave filter is configured to have a balanced-to-unbalanced conversion function such that both ends of the middle IDT 1 of a longitudinally coupled resonator-type surface acoustic wave filter element 6 including three IDTs 1, 2, and 3 and two reflectors 4 and 5 are connected to balanced signal terminals 11 and 12, respectively, and such that one end of each of the left and right IDTs 2 and 3 is connected to an unbalanced signal terminal 13 through an IDT 7 of a surface acoustic wave resonator 10 in which reflectors 8 and 9 are disposed on either side of the IDT 7. In the surface acoustic wave filter, the other ends of the IDTs 2 and 3 are connected to a ground terminal.
The element chip 30 is housed in a package which can be divided into an upper portion and a lower portion in the bottom-side portion. FIG. 7 shows the upper surface of the upper portion 33 of the package bottom portion 31 in which the element chip 30 is mounted, FIG. 8 shows the upper surface of the lower portion 36 of the package bottom portion 31, and FIG. 9 shows the lower surface (bottom surface of the package) of the lower portion 36 of the package bottom portion 31.
As shown in FIG. 7, wiring patterns (lands) 42 to 45 are exposed in the die attachment portion 41 of the upper portion 33 of the package bottom portion 31 and bump-connected to the terminals (pads) of the element chip 30 by a bump 39, shown by a white circle in FIGS. 6 and 7. In FIG. 7, via holes 46 and 47, shown by black circles, pass through the upper portion 33 of the package bottom portion 31 and the wiring patterns 45 and 44 and wiring patterns 61 and 63 of the lower portion 36 shown in FIG. 8 are connected. Among external terminals shown in FIG. 9, the right middle external terminal 56 is an unbalanced signal terminal, the left upper and lower external terminals 52 and 53 are balanced signal terminals, and the other external terminals 54 and 55 are ground terminals. The external terminal 56 defining an unbalanced signal terminal is connected to the unbalanced signal wiring pattern 42 through a castellation 48. The external terminals 52 and 53 defining balanced signal terminals are connected to the balanced signal wiring patterns 43 and 44 through castellations 49 and 50.
Finally, corresponding to the locations of the first and second balanced signal terminals (pads) 11 and 12 on the element chip 30 shown in FIG. 6, as shown in FIG. 7, in the flip-chip mounting package of the element chip 30, the first balanced signal terminal wiring pattern (pad) 43 is disposed in the middle of one side of the package, and the second balanced signal terminal wiring pattern (land) 44 is disposed in the corner portion close to the first balanced signal terminal wiring pattern (land) 43. In the element chip 30, a signal line 1a connecting one end of the IDT 1 and a first balanced signal terminal 11 and a signal line 1b connecting the other end of the balanced ITD 1 and a second balanced signal terminal 12 are asymmetrically disposed, and thus, the balancing is deteriorated. Then, as shown in FIG. 9, the external terminals 52 and 53 as the first and second balanced signal terminals are disposed so as to be symmetrical around the central axis of the package, and the balancing is adjusted by altering the path difference in the package between a signal line connected to the external terminal 52 defining the first balanced signal terminal and a signal line connected to the external terminal 53 defining the second balanced signal terminal (see, for example, Japanese Unexamined Patent Application Publication No. 2002-271168).
The surface acoustic wave filter package shown in FIGS. 6 to 9 can be also used for mounting an element chip 60 including two longitudinally coupled resonator-type surface acoustic wave filter elements 66 and 68 having three IDTs 66a, 66b, and 66c, and 68a, 68b, and 68c and two reflectors 66s and 66t, and 68s and 68t, respectively, as shown in FIG. 22. That is, the element chip 30 shown in FIG. 1 and the element chip 60 shown in FIG. 22 have the same external dimensions and the same terminal (pad) configuration.
In Japanese Unexamined Patent Application Publication No. 2003-204243, a float balanced-type surface acoustic wave filter in which the balancing is improved such that two terminals extending on either side in the direction perpendicular to the surface acoustic wave propagation direction of the middle ITD of a longitudinally coupled resonator-type surface acoustic wave filter element are connected to balanced signal terminals, IDTs on both sides are connected to unbalanced signal terminals by using two unbalanced signal lines, and one balanced signal line and one unbalanced signal line intersect in three dimensions through an insulating film is disclosed.
As in the related example shown in FIG. 5, when two elements are cascade-connected and a ground pad is disposed between stages, since stray capacitance between the wiring for the cascade connection and the ground pad is large, there is a problem in that the insertion loss in the passband is large.
Furthermore, in a surface acoustic wave filter having a balanced-to-unbalanced conversion function which includes balanced signal terminals that are connected to the terminals on both sides of one IDT as disclosed in Japanese Unexamined Patent Application Publication No. 2002-271168), since the structure of the package is complicated and specific, the configuration of the package is limited to the element structure.
Accordingly, for example, a surface acoustic wave filter 70 shown in FIG. 10 includes two longitudinally coupled resonator-type filter elements 71 and 72 having three IDTs 71a, 71b, and 71c, and 72a, 72b, and 72c and two reflectors 71s and 71t, and 72s and 72t that are cascade-connected, one end of the middle IDT 71a of one longitudinally coupled resonator-type filter element 71 is connected to an unbalanced terminal 73, and one end (one bus bar) of the middle IDT 72a of the other longitudinally coupled resonator-type filter element 72 is divided into two bus bars and the two bus bars are connected to balanced signal terminals 74 and 75.
Another surface acoustic wave filter 80 shown in FIG. 11 includes two sets of surface acoustic wave filter elements 81, 82, 83, and 84 having three IDTs 81a, 81b, and 81c; 82a, 82b, and 82c; 83a, 83b, and 83c; and 84a, 84b, and 84c and two reflectors 81s and 81t; 82s and 82t; 83s and 83t; and 84s and 94t that are cascade-connected, one end of each of the middle IDTs 81a and 83a of the surface acoustic wave filter elements 81 and 83 is connected to balanced terminals 85 and 86, and one end of each of the other surface acoustic wave filter elements 82 and 84 is connected to an unbalanced terminal 87. However, with this configuration, other surface acoustic wave filters cannot share the package with a surface acoustic wave filter having a balanced-to-unbalanced conversion function of another structure different in terms of the way surface acoustic wave elements are coupled.
Moreover, since the signal lines inside the package are asymmetrical, the affect of parasitic capacitance, is different between balanced signal terminals, and, as a result, there is a problem in that the balancing between balanced signal terminals is deteriorated.
In a surface acoustic wave filter in Japanese Unexamined Patent Application Publication No. 2002-204243, an intersection is performed by providing a balanced signal line on a piezoelectric substrate and providing an unbalanced signal line on an insulating film disposed on the balanced signal line. Accordingly, the difference between the parasitic capacitance and bridge capacitance entering the two balanced signal terminals increases and the balancing cannot be sufficiently improved.